This invention relates to vehicles, such as sport utility vehicles, having a liftgate for access to a cargo compartment and more particularly to a counterbalance system for the liftgate.
Vehicles that have liftgates usually include a counterbalance system that stores energy when the lift gate is closed with the stored energy then being used to assist in the subsequent lifting of the liftgate to an open position. A common liftgate counterbalance system uses a pair of gas springs that are pivotally attached to opposite sides of the lift gate at one end and to the vehicle body at the opposite end.
A drawback with gas springs is that the gas springs are sensitive to variations in ambient temperature. This results in the use of gas springs that resist closure of the liftgate with considerable force on hot days. For instance, the gas spring or springs must be strong enough to open the liftgate on the coldest day (usually assumed to be xe2x88x9240xc2x0 C.) Such gas springs increase closing resistance substantially on the hottest day (usually assumed to be 80xc2x0 C.) Therefore considerable effort must be used to close the liftgate or a very large electric motor used in the case of a power operated system.
Liftgates that have two or more gas springs for a counterbalance system are common. These gas springs generally occupy a position in which their axes is substantially parallel to the liftgate so that the gas springs are hidden when the liftgate is closed. In this closed position the moment arm of the gas springs is quite small. With such systems the liftgate may move about one-third of its total travel range before the ga cylinders exert sufficient force to open the liftgate further without the application of an independent lifting force. There are even some systems in which the gas springs pass over center and bias a liftgate toward a closed position when the liftgate is closed. With these self-closing systems a liftgate may need to be more than one-third open before the gas springs will open the liftgate further. Thus the geometry of the gas spring counterbalance system itself increases the drawback of gas spring counterbalance system.
Decklids have been counterbalanced with steel coil springs for many years. A decklid when open, with spring relaxed has the gravity moment at its minimum. As the decklid is closed the gravity moment and the spring output both increase. With spring and gravity moment tracking together, counterbalancing a decklid is straightforward. The difficulty with counterbalancing a liftgate, in comparison to a decklid is that with the liftgate in the open position, and the counterbalance spring relaxed, the gravity moment is near its maximum. This means that when the spring is at its minimum output the load from the liftgate is maximum. The converse is also true. When the spring is at a maximum output the liftgate has its smallest gravity moment. Thus coil spring counterbalance systems for decklids are not well suited for liftgates.
The counterbalance system of this invention uses a compression spring or springs as an alternate for gas springs in a liftgate application and thus provides a liftgate counterbalance system that is not sensitive to variations in ambient temperature. The counterbalance system of the invention also has an improved geometry and changing mechanical advantage for applying the compression spring forces of the counterbalance system to assist in opening the liftgate.